In this type of a conventional X-ray computed tomography, as shown in FIG. 1 by a solid line, a projectional distribution of X-ray is measured by X-ray detector c in such a way as that an X-ray beam is projected from X-ray source a toward X-ray tested tissue b. Then, as shown in FIG. 1 by a dotted line, X-ray source a and X-ray detector c are rotated by a desired angle of .alpha. (for example 1.degree.), and at this position the X-ray beam is projected again toward X-ray tested tissue b, a projectional distribution of X-ray under the rotation of an angle of .alpha. is measured again, and several (60 to 360) times similar operations are performed. Thereafter, data provided by these many projectional distributions of X-ray are calculated by Fourier transform or a convolution method, and a tomographic image of the X-ray tested tissue b is reproduced on the basis of a result of the processing.
However, in this type of a conventional X-ray computed tomography, it is necessary to have many projectional distributions of X-ray in order to reproduce a tomographic image of X-ray tested tissue b, resulting in providing some problems as follows:
(1) Since a long period of time is needed (several seconds to several minutes) for sampling data, it is not possible to reproduce a tomography of a moving tested tissue b. PA1 (2) Exposure to X-ray shows a high value, resulting in that some bad effects might be applied to the tested tissue when the tested tissue b is a living thing such as a human body. PA1 (1) Since it is possible to reconstruct a tomographic image of X-ray tested tissue B from a single projectional distribution D, an extremely short period of time for sampling data (less than 30 millisecond) may be made and therefore it is also possible to have a reconstruction of a clear tomographic image of a moving X-ray tested tissue B (for example, a heart). PA1 (2) Since X-ray exposure is extremely small (several tenths or several hundredths compared to a conventional method), no bad effect may be provided even if X-ray tested tissue B is a living thing. PA1 (3) Since it is possible to calculate each of the X-ray absorption coefficients of u picture elements for each of the groups of picture elements constituted by the number u pieces of picture elements in reference to a group of measured values constituted by the number of measured values greater than the number u, it is possible to decrease substantially the data required for one processing operation, simplify substantially the data processing, and make a data processing apparatus small and low cost. PA1 (4) Since a tomographic image of X-ray is reconstructed in reference to the number of measured values greater than the number of picture elements u constituting a group of picture elements forming a parallel plane of a tomographic plane containing X-ray tested tissue B, it is possible to provide a substantial increase of precision of reconstruction.
The present invention is provided to overcome these disadvantages and its object is to provide a reconstruction method of reconstructing an X-ray computed tomographic image, in which a reconstruction image having a high accuracy or precision may be produced by using a single projectional distribution of X-ray as data for reconstructing a tomographic image of the tested tissue.